An Introduction to Unix Permissions -- Part Two

09/13/2000

In last week's article, we looked at the three base permissions and the three specialty permissions. This week, we'll be using chmod to change permissions. You might want to reread last week's article, as we'll be implementing the concepts it introduced.

But first, never, ever, ever change the default permissions on the files
and directories that came with your FreeBSD system. The creators of
FreeBSD understand permissions; unless you are a systems administrator or
a security engineer with a good reason to change these defaults, please
leave well enough alone.

Instead, we'll be creating test directories and test files to practice
with. Once you're comfortable with setting permissions, you'll be able to
create your own directories and set their permissions according to your
needs.

Let's start by taking a look at the chmod command. There are two modes of operation for chmod: absolute mode (which uses numbers) and symbolic mode (which uses letters).

In absolute mode, chmod uses four numbers to represent the following four sets of permissions:

Specialty permissions (SUID, SGID, directory sticky bit)

Base permissions for the owner of the file (rwx)

Base permissions for the primary group of the file (rwx)

Base permissions for everybody else (rwx)

Instead of using the letters r, w, x, s, or t, it uses the numbers 4, 2, and 1 in this order:

421 421 421 421

In the first set of numbers, 4 = SUID, 2 = SGID, and 1 = directory sticky bit. In the next three sets of numbers, 4 = read, 2 = write, and 1 = execute; again the order is owner, group, and everyone else. If a permission is to be denied, a 0 is used, not the - symbol.

So, if I wanted to to set the SUID bit on a file, give its owner full access, and give the primary group and everyone else read and execute access, I
would want permissions like this:

400 421 401 401

To set this using chmod, I must first total each set of permissions like so:

400 = 4+0+0 = 4421 = 4+2+1 = 7401 = 4+0+1 = 5401 = 4+0+1 = 5

so I can tell chmod this:

chmod 4755 name_of_file

Let's try this and see if it works. As a regular user, cd into your
home directory and create a test file. In this example, I am logged in as
the user genisis; if I type cd without any arguments, I will be taken to genisis' home directory.

Note that whoever creates a file becomes the owner of the file; the
primary group of the file will be the primary group of the owner of the
file. This is important as only the owner of the file (and root) can
change the permissions of a file.

Now, how would you use chmod to change the file back to its original permissions? Let's see if we can do the math and get it to work. Our
original permissions looked like this:

-rw-r--r-- 1 genisis wheel 0 Aug 19 11:27 test

There isn't an s, S, t, or T in the original permissions, so the first set
will be 0 + 0 + 0 = 0.

The owner has rw which is 4 + 2 + 0 = 6.

The group has r which is 4 + 0 + 0 = 4.

Everyone else has r which is 4 + 0 + 0 = 4.

So let's see if the following works:

chmod 644 test
ls -la test

-rw-r--r-- 1 genisis wheel 0 Aug 19 11:27 test

Note that the following command would yield the same result:

chmod 0644 test

However, you can omit the 0 if it represents the specialty permissions.

Now let's try chmod in symbolic mode. The syntax for symbolic mode is a bit longer:

chmod who operator permission filename

where who can be:

u = user (owner)

g = group

o = others

a = all, or ugo

and operator can be:

+ add this permission

- take away this permission

= make this permission equal to

and the permissions can be r, w, x, s, t, and X. Note that there is no
capital S; if you want to keep the execute bit when you set the SUID or SGID bits, use both s and x. If you want to set the SUID bit, use s with a who of u; to set the SGID bit, use s with a who of g. To set the sticky bit, use t with a who of o.

We got a syntax error on that command because chmod expects the who, operator, and permissions to be a string of characters without any spaces. Whatever follows the space is interpreted as the name of the file whose permissions are being set. Let's try again: